This invention relates to a method of alignment in an optoelectronic assembly, in particular, but not exclusively, to a method of generating an alignment feature in an optoelectronic assembly which enables another part, for example an optical fibre, to be aligned to a device within the assembly.
Conventionally, a micro fabricated optoelectronic device, for example a single chip photodetector or laser source device, is packaged within an associated hermetically sealed package for protection. An optical fibre for conveying electromagnetic radiation to or from the device is attached to the package and penetrates through the package to contact directly onto the device. Alternatively, the optical fibre penetrates through the package to align remotely to the device, a secondary structure physically connecting to the device and receiving an end of the fibre thereby securing the fibre in alignment with the device. Alignment of the optical fibre to the device can often be difficult to achieve especially when the fibre is a monomode fibre having an associated core diameter in the order of a few ricrometres. When the device is a III-V compound device, for example a gallium-indiumarsenide laser source, alignment of the fibre to the device can be critical; an alignment error in the order of 0.1 xcexcm can adversely affect coupling efficiency of the device to the fibre. There also arises an issue of mechanical stability where the optical fibre abuts onto the device; relative movement therebetween over time of tens of nanometres can adversely effect transmission efficiency from the device into the fibre or detection efficiency of the device to incoming fibre-borne radiation.
There are a number of conventional approaches to assist with aligning optical fibres to associated devices.
In a first conventional approach, there is provided an optoelectronic device housed within a hermetically sealed package, the device having etched thereinto a channel for receiving a core of an optical fibre, the fibre passing from a region exterior to the package through the package to terminate on the device. Assembly of the fibre to the device requires considerable operator skill and damage can occur to the device if the fibre is misdirected during assembly, for example the fibre scraping and severing metal electrodes of the device. Location of the core in the channel can occur by mechanical abutment although optically transparent bonding agents, for example Norland Inc. optically transparent UV curing adhesive, can be advantageously added to obtain a robust joint.
In a second alternative approach, an epitaxial alignment structure is formed onto the device to provide lateral abutment edges onto which the core can register. However, this second approach suffers the same disadvantages of the first approach in that a skilled operator is required for manipulating the core to align it precisely to the device without causing damage thereto.
In a granted U.S. Pat. No. 4,892,377, the inventor discloses an approach to accurately align an array of optical fibres with corresponding optical components such as waveguides. The fibres are fixed in accurately etched V-grooves formed into a substrate connected to the optical components and can be secured thereto using solder. Such an approach requires skilled operators to manipulate the fibres into the V-grooves to obtain a satisfactory alignment providing acceptable matching.
The inventor has appreciated that it is desirable to accurately align fibres to devices without having to physically bond or abut the fibres to the devices. Superficially, such an approach would seem unworkable because each fibre would have to be accurately aligned to an associated intermediate region and a corresponding device would also have to be accurately aligned to the intermediate region thereby fixing the fibre spatially with respect to the device; this would result in a build up of tolerances which would be more difficult to control than the first and second conventional approaches described above.
The inventor has evolved a method of alignment in an optoelectronic assembly which addresses alignment problems associated with spatially defining a fibre""s position with respect to an associated device within the assembly without having to bond the fibre directly to the device within the assembly as in the prior art.
According to a first aspect of the present invention, there is provided a method of alignment in an optoelectronic assembly, the assembly including one or more optoelectronic devices and interfacing means for interfacing from the one or more devices to one or more corresponding optoelectronic components external to the assembly, the method characterised in that it includes the steps of:
(a) defining one or more regions of the interfacing means to which the one or more devices are responsive to or emissive towards, thereby rendering the one or more regions delineated for processing purposes;
(b) processing the assembly to generate one or more alignment features at the one or more regions, the one or more features operable to assist with aligning the one or more external components relative to their respective one or more devices within the assembly;
(c) aligning the one or more components to their respective features so as to be operable to emit towards or receive radiation from their respective one or more devices; and
(d) applying attaching means for attaching the one or more external components to the assembly when aligned to their respective features, the one or more components thereby optically aligned to their corresponding one or more devices within the assembly.
The method provides the advantage that the one or more devices are capable of defining positions of their respective alignment features in the interfacing means, thereby assisting to align the one or more external components to the devices without the position of the interfacing means needing to be initially precisely defined with respect to the one or more devices.
Advantageously, in step (a), the interfacing means includes a wall of the assembly, the wall bearing a system of layers responsive to radiation emitted from the one or more devices within the assembly for defining the one or more regions in the system of layers, the layers providing a template for the formation of the alignment features in step (b). Inclusion of the system of layers enables the one or more devices to define positions of their corresponding one or more alignment features, the system of layers responsive to radiation emitted from the one or more devices.
Conveniently, the system of layers comprises:
(a) an etch resist layer through which the wall is processable to generate the one or more features therein; and
(b) a photochromic layer operable to be activated by externally applied radiation and capable of being rendered locally transmissive in response to radiation received thereat from the one or more devices,
thereby enabling the externally applied radiation to define regions in the resist layer corresponding to the one or more features, the resist layer providing a template for formation of the one or more features. Use of the photochromic layer provides a simplified method of alignment which does not need to be conducted in darkroom conditions.
Alternatively, the system of layers includes multiplying means for frequency multiplying radiation emitted from the one or more devices within the assembly, thereby generating corresponding relatively shorter wavelength radiation for defining the one or more regions in one or more layers of the system. Inclusion of the multiplying means enables one or more of the devices emitting infra-red radiation having a wavelength in the order of 1300 to 1550 nm to form a latent image in the system of layers, the latent image processable for ultimately forming a template in the system of layers through which the wall can be etched to generate the alignment features therein.
The multiplying means preferably comprises a multiplying layer including potassium titanyl phosphate which is operable to emit radiation at a relatively shorter wavelength when stimulated by relatively longer wavelength radiation emitted from the one or more devices. Use of potassium titanyl phosphate material enables infra-red radiation output from the one or more devices within the assembly to be transformed to radiation within the visible electromagnetic radiation spectrum to which other of the layers of the system are responsive.
In order to provide adequate spatial resolution for delineating the alignment features, the multiplying layer beneficially comprises a continuous film of potassium titanyl phosphate.
Advantageously, the system of layers includes a photoresponsive layer responsive to radiation emitted from the one or more devices within the assembly subject to frequency multiplication in the multiplying means, the photoresponsive layer processable to define a first template which is transferable to an etch resist layer of the system, the etch resist layer forming a second template through which the wall is processable to generate the one of more features therein. Use of the photoresponsive layer enables radiation within the visible range to define features in the photoresponsive layer which can be transferred from that layer by ultra violet (UV) radiation exposure to a photoresist layer, the photoresist layer being required to withstand etching gases or solutions where alignment features are not to be formed into the wall.
Conveniently, where direct responsivity to infra-red radiation is required in the system of layers, the system includes a photoresponsive layer directly responsive to radiation emitted from the one or more devices within the assembly, the photoresponsive layer processable to define a first template which is transferable to an etch resist layer of the system, the etch resist layer forming a second template through which the wall is processable to generate the one of more features therein.
The method of the invention according to the first aspect described in the foregoing is modified where the assembly incorporates one or more devices which are not radiation emissive. Preferable, the interfacing means includes a wall of the assembly, the wall bearing a system of layers responsive to radiation received thereon from a source external to the assembly, the radiation from the source being guided by a response from one or more of the devices within the assembly to define the one or more regions in the system of layers, the layers providing a template for the formation of the one or more features in step (b). Use of the external source compensates for the one or more devices being responsive to radiation but not radiation emissive.
Conveniently, the system of layers includes a photoresponsive layer responsive to radiation emitted from the external source, the photoresponsive layer processable to define a first template which is transferable to an etch resist layer of the system, the etch resist layer forming a second template through which the wall is processable to generate the one of more features therein. The photoresponsive layer can, for example, be a photoemulsion responsive to radiation emitted from the external source, the source not restricted to being emissive at infra-red radiation wavelengths in the order of 1300 to 1550 nm.
Beneficially, the etch resist layer includes an UV-responsive organic resist layer. Such resist layers are conventionally used in semiconductor fabrication processes for providing a template for etching processes.
Where problems of resist adhesion and robustness are experienced, for example when undertaking isotropic acid etching using buffered hydrofluoric (HF) acid, the etch resist layer advantageously further includes a UV-responsive organic resist layer and also a silicon nitride layer into which the second template in the organic resist layer is transferable to form a third template in the silicon nitride layer through which the one or more features are generated. Silicon nitride is better able to resist etchant attack than organic resist for many conventionally used silicon etchants.
Conveniently, there are several alternative etching processes which can be used for forming the alignment features in the wall; the features can be generated by one or more of anisotropic wet etching, isotropic wet etching, dry plasma etching or dry reactive ion etching processes. Preferably, the one or more features are recesses into which the one or more external components are registerable for aligning to their respective one or more devices within the assembly.
Where the assembly is used for one or more of receiving and emitting infra-red radiation in the order of 1300 to 1550 nm wavelength, the wall is conveniently fabricated from silicon transmissive to infra-red radiation.
For ease of fabricating the assembly, the attaching means is advantageously a substantially transparent UV-curable adhesive.
In a second aspect of the present invention, there is provided an assembly fabricated by a method according to the first aspect of the invention, the assembly characterised in that it comprises one or more optoelectronic devices incorporated within a housing, the housing including the interfacing means in the form of a wall, the wall including the one or more features to which the one or more external components are registerable for aligning the components to their respective one or more devices.
Conveniently, the wall is fabricated from  less than 100 greater than -cut single-crystal silicon and is wet anisotropically etchable to form pyramidal recesses therein for providing the one or more features to which the external components are registerable. The pyramidal features provide a beneficial characteristic that they are self-limiting in size in a wet anisotropic etching process, thereby counteracting a need to monitor etching critically to counteract overetching, for example as can occur when wet isotropic etchants are employed.
Preferably, in order to obtain a more ruggedized assembly, the wall can be recessed within the housing, thereby exposing a greater surface area of the housing to which the attaching means can bind for anchoring the one or more external components to the assembly.
Conveniently, the one or more external components include one or more optical fibres, each fibre having its core protruding from its cladding where the core is registerable into the one or more features. Such alignment of the cores to the features provides an enhanced degree of coupling efficiency between the one or more external components and corresponding one or more devices within the assembly.
In a third aspect of the present invention, there is provided an array of assemblies, each assembly according to the second aspect of the invention.